In recent years, separation membranes have been in use in diverse fields, encompassing, among other things, water treatment, including drinking water production, water purification treatment and wastewater treatment, and the food industry. In the water treatment field, including drinking water production, water purification treatment and wastewater treatment, separation membranes are used to remove impurities from water as an alternative to conventional sand filters and coagulation precipitation systems. In the food industry, separation membranes are used for the separation and removal of yeast utilized in fermentation and concentration of liquids.
Areas in which separation membranes, capable of a selective separation of components from liquid mixtures, are widely used include: the manufacture of ultrapure water, desalination of seawater or brackish water, establishment of a closed industrial water supply system based on the removal or separation/recovery of components from dyeing or electrodeposition painting effluent, and concentration of active ingredients in the food industry.
In concrete terms, separation membranes in which a separation functional layer comprising a cross-linked polyamide obtained through an interfacial polycondensation reaction between a polyfunctional amine and polyfunctional acid derivative (e.g. a chloride) is deposited over a porous film are a focus of attention because of their high permeability and selective separation performance. From an economic viewpoint, separation membranes that are in use in various fields are required to have excellent permeability. This is because excellent permeability is conducive to compact equipment and savings in plant costs and advantageous in terms of the cost of membrane replacement and size of the required site area.
However, practical requirements for separation membranes have been growing in sophistication each year, and this has given rise to calls for the development of high-permeability separation membranes that tolerate low-pressure operation while maintaining high solute removal performance from the viewpoint of energy conservation. In response to such calls, it has been proposed to remove acidic substances, produced in an interfacial polycondensation reaction, outside the system through the addition of compounds such as potassium hydroxide and trisodium phosphate, addition of an acylation catalyst or addition of compounds with a solubility parameter of 8 to 14 (cal/cm3)0.5 as a means to achieve high permeability (see patent documents 1 to 4). Other known methods relating to a separation membrane featuring a cross-linked polyamide polymer include contact treatment with a chlorine-containing aqueous solution (see patent document 5) and contact treatment with a nitrous acid-containing aqueous solution (see patent document 6).